“lock down” a property value after it has been entered.

You should be able to configure a rule in Metadata Card Configuration where you can make a property Read Only when certain filter conditions are met. It is up to you to define the conditions. It could be as simple as the same property being "IsNotNULL" in which case the value cannot be changed after it has been saved the first time.

From what I can tell it doesn't allow me to use a date property in this way.  The Filter>Property field only lets me use "choose from list" or "Boolean" .  Thanks for the idea.

You are right, I had not checked that part. Sorry.

If the "Need by Date" is within a specific group, you could make the Group read-only once it goes into the next state?

The only limitation is that every property within the same Group will also become read-only...